DESCRIPTION (Applicant's Description): The c-myc oncogene is associated with a variety of malignancies including Burkitt's lymphoma and certain leukemias. While c-myc appears necessary for the development of many malignancies, it is not sufficient. For this reason, attempts to identify supporting oncogenes that cooperate with c-myc have been undertaken. One oncogene that has been identified using the technique of retroviral insertional mutagenesis (proviral tagging) is bmi-1 (B cell -specific Mo-MLV integration site 1). The proviral tag frequently integrates at bmi-1 in E(mu)-myc transgenic mice. Integration at this site decreases the latency of tumor development from around 150 to about 50 days with uniform fatality in all animals, suggesting a potential interaction between bmi-1 and c-myc. Despite this observation, the precise function of bmi-1 is unknown. The investigators' preliminary results suggest that bmi-1 has oncogenic properties in collaboration with c-myc in vitro. bmi-1 is able to rescue cells from c-myc driven apoptosis. Additionally, bmi-1 is able to suppress transcription in a chimeric GAL4 system. Phase I of this project is designed to further delineate specific regions required for each of the above findings. bmi-1 mutants with targeted deletions and mutations of selected sites will be prepared and analyzed for their ability to transform cells in a rodent fibroblast assay, protect against apoptosis using immortalized rodent fibroblasts, suppress transcription in a chimeric GAL4 system and localize bmi-1 to the nucleus. Moreover, Drosophila homologs of bmi-1, which have also been shown to suppress transcription in transient transfection systems, will be assayed to determine if they share similar biologic capabilities of transformation and apoptosis attenuation. Phase II of this project is designed to determine protein partners of bmi-1 by use of the yeast two-hybrid system. Additionally, based on preliminary results suggesting DNA-binding, specific DNA binding motifs will be identified. The final portion of the Phase II studies will focus on the expression of bmi-1 in human malignancies, making use of the known chromosomal localization to 10p13 (a region often translocated in a variety of malignancies including infant ALL and T-cell lymphomas). These studies should clarify the role of bmi-1 in cancer development.